1. Field of the Invention
The present invention relates to myocardial imaging, and in particular to myocardial image registration, and more particularly to myocardial image registration using non-rigid transformations such as a global affine transformation.
2. Background of the Invention
Narrowing of the vessels which feed the muscle of the heart can lead to heart attacks, a condition in which the heart muscle, deprived of oxygen and nutrients "dies". One of the methods to check if a patient has narrowing of those vessels is a myocardial perfusion scintigraphy. This is a test in which the patient is injected with a tracer (or dye) that goes to different regions of the heart muscle in proportion to the blood flow that goes to those regions. Radioactive emissions from the tracer are monitored and by triangulation the concentration of the tracer in various regions is determined. The test is performed in two parts. First the tracer is injected while the patient is resting and a myocardial image (a "rest image") is taken. Then, a second tracer injection is made while the patient is exercising, for instance walking on a treadmill for approximately 20 minutes, and another myocardial image (a "stress image") is taken. Each myocardial image is a two-dimensional picture of the heart which shows the distribution of the blood flow to the various parts of the heart muscle.
For healthy hearts this blood distribution is "normal" (i.e., similar to a template image obtained from averaging images from a large number of healthy hearts) for both the resting and stress images. In cases where there is a narrowing of one of the blood vessels, the distribution is normal after the resting injection but abnormal after the exercise injection. These "transient defects" are detected by a comparison of the stress and rest images. In other cases, such as after a heart attack, because some muscle tissue has died, or if the narrowing of blood vessels is severe, both the resting and stress images will be abnormal. These "fixed defects" are therefore detected by comparison of the rest image and the template image.
Comparisons of the stress image and the template, or the stress and rest images, are not easily made visually. To improve the accuracy of the test, quantitative comparison have been proposed. However, this is problematic due to the large natural variation in the orientation, size and shape of different human hearts, even between "normal" hearts. This difficulty has been overcome to some extent by strategies to eliminate the effects of variations in orientation, size and shape, but most of these strategies lose important information in the process of the simplification of the image data.
Myocardial perfusion scintigraphies are the third most frequently performed nuclear medicine procedure (after bone and lung) in the United States. Their role in the patient management is crucial, since they are used to select for intervention (coronary arteriography) those patients suspected of having coronary artery disease, and in some cases to determine the significance of known coronary lesions. At present the sensitivity and specificity of the method are 52% to 99% and 55% to 93%, respectively. As shown in Table 1, the variation is due in part to interpretation differences (e.g. the inclusion of transient defects in the interpretation). But apart from fundamental methodological limitations and interpretation differences, the goal of the present invention is to decrease the normal variability using an automated registration method. A small increase in the sensitivity and specificity will represent a large number of patients in whom the correct diagnosis will be made.
The analysis of scintigraphic myocardial perfusion images is generally based on some form of a polar transformation where the myocardial densities are sampled along evenly spaced radii projecting from an origin in the center of the cavity. For instance, in a method termed "radial sampling" (Goris 87) the three-dimensional images are sampled along rays or radii originating in the center of the left ventricular cavity. Each radius is characterized by a longitudinal angle "a" and latitudinal angle "b". The origin of "b" is the apex of the myocardium, and "b" goes in 32 steps from 0 to 135 degrees (generally the myocardium does not extend beyond 135 degrees). The origin of "a" is the middle of the lateral wall, and "a" varies from 0 to 360 degrees. The maximum count rate density along the radius R(a,b) is stored in the 64.times.64 matrix B at the location (K,L) where K and L are defined as: EQU K=(32b/135) cos (a)-32 EQU L=(32b/135) sin (a)-32
The matrix B is therefore a planar image in which the central points represent locations near the apex of the myocardium, and the peripheral points represent the locations near the base of the myocardium. FIG. 1 displays the central horizontal 100 and vertical 150 long axis slices of a myocardium. The long axis of this myocardium is oriented from top to bottom in FIG. 1, with the apex 105 pointing towards the bottom of the page. On each slice 100 and 150 a number of exemplary search rays 110 are shown, and broken lines 120 lead from the end of each search ray 110 to its location on a bull's eye map 180. As shown in FIG. 1, the anterior wall is mapped on top of the image, the inferior wall is mapped to the bottom of the image, and the septum is mapped to the left-hand side of the image. This matrix B is comparable to the bull's eye maps described in the literature (Garcia 81, 85, Maddahi 89, Mahmarian 90, Van Train 82, 86, 90, Vogel 80), except for the fact that the distance from the center represents a latitude "b" in three dimensions, rather than a short axis plane position: The bull's eye map approach interrogates the volume as if it were a set of planes (Goris 86).
Since a polar transform only measures angles, all morphological attributes of the myocardium are reduced to angular coordinates. This provides for a normalization of size and shape (Goris 82) and facilitates the comparison of target cases to a population of normal cases, and the comparison of target cases. This is illustrated by FIG. 2 which shows the myocardium 210 and 220 from two patients. Although the myocardia 210 and 220 differ considerably in size, as reflected in the count density cross-sections 230 and 240 shown in FIGS. 2B and 2C, the corresponding polar maps 250 and 260 of FIGS. 2D and 2E, respectively, are very similar. This is also true for the polar component in bull's eye mapping, in which the third axis is divided in a set number of parallel "thick slices" between the apex and the base of the heart.
There are four significant limitations inherent in polar transformations, of which two are alignment limitations (Goris 82); one is due to the sampling direction and one is due to the inherent assumption that there is no relevant morphology except angular morphology. One alignment issue associated with such polar maps is the placement of the origin of the rays, as illustrated by FIGS. 3A-3D. FIGS. 3A and 3B display the same myocardial image, but in FIG. 3A the origin 311 of the rays is displaced slightly to the left relative to the origin 321 of the rays in FIG. 3B. The resulting polar maps 330 and 340 of FIGS. 3C and 3D, respectively, clearly differ considerably due to this displacement of the origin. The placement of the origin is important since structures closer to the origin are relatively oversampled. Therefore if the origin is moved towards a perfusion defect, that perfusion defect will appear larger.
Another alignment issue associated with such polar maps is the orientation of the poles of the polar coordinate system, as illustrated by FIGS. 4A-4D. FIGS. 4A and 4B display the same myocardial image, but in FIG. 4A the myocardium is rotated clockwise slightly relative to FIG. 4B. The resulting polar maps 430 and 440 of FIGS. 4C and 4D, respectively, clearly differ considerably due to this rotation, illustrating that if the zero angle is misplaced identical distributions will appear different from each other.
Another problem with such radial mapping methods is that if the lesion is relatively small and not well aligned in relation to the sampling ray direction, the lesion may be underdetected. For instance, as shown in FIG. 5 the defect 510 on the left side of the myocardium 500 is somewhat smaller than the defect 520 on the right side of the myocardium 500. The longitudinal axis of the lefthand defect 510 is well-aligned with the rays coming from the origin 501 so this defect will be clearly represented in a polar map. However, the longitudinal axis of the right-hand defect 520 is not well-aligned with the rays coming from the origin 501 so samples along these rays will include healthy myocardium tissue or will exclude defective myocardium tissue, and this defect will not be clearly represented in a polar map. This sampling direction problem is only partially overcome by modifying the method so that the rays are normal vectors to the myocardium surface, since the direction of the defect across the myocardium is variable and may not be parallel to such normal vectors.
Another problem with such radial mapping methods is that the proportion of different parts of the myocardium are not necessarily fixed. For instance, the base of the heart with the papillary muscles may be more or less prominent. As shown with the two superimposed myocardia 610 and 620 or FIG. 6, because there may be relevant non-angular morphologies, equal angular coordinates do therefore not necessarily map into identical myocardial structures. It may be noted that other registration techniques, such as aligning myocardia 710 and 20 by aligning best fitting ellipsoids 730 as shown in FIG. 7, do not provide significantly better results.
Alignment problems are mostly caused by operator variability in identifying the long axis and/or the center point, and attempts have been made to overcome this problem by using principal axis orientation (Germano 1995). It can be shown however that alignments based on the principal axes of simple geometric forms fitted to the myocardial surfaces does not guarantee stable center points relative to the organ or a stable anatomical alignment (Slomka 1995).
A method is therefore needed which allows registration of a target image automatically on a template image whose shape and size may be different, though both target and template have similar morphological features. If this template is well oriented, the target image will be well oriented, and any small deviation from the ideal orientation will be constant across all target images. Following the registration, predefined myocardial segments in the template image can be transferred exactly to the target image. As a result, size and shape normalization are obtained without the need for radial sampling, and regional count rate distributions can be compared from case to case.
The present invention is directed to a method for elimination of normal differences in orientation, size and shape by a technique to be termed "elastic image registration." Elastic image registration provides a transformation similar to the "morphing" transformations which are becoming commonly used in the television and film entertainment industry. According to the present invention, a myocardial image is transformed by elastic image registration to substantially overlap a template heart (obtained by the averaging of many normal hearts) so that the differences between the template heart and the transformed image represents important differences in regional blood flow. The morphing transformation of the present invention involves the recognizing of myocardial features (for instance, by the use of an eight-dimensional coordinate system used for distance calculations), centering of the myocardial image, the resizing and changing of the proportions of the myocardial image, and fine shape adjustments.
Myocardial perfusion scintigraphy plays an important role in the diagnosis of patients which are suspected of having a narrowing of the blood vessels in the heart. Although in many cases the symptoms and the findings of a rest/stress comparison can unambiguous, in some cases the diagnosis cannot be made with certainty so a more definitive test, such as a coronary arteriogram, would be required. However, coronary arteriograms are expensive, unpleasant for the patient, and carry some risks for the patient, so a method of increasing the accuracy of diagnosis with myocardial perfusion scintigraphy would be desirable.
The method of the present invention for registration of images from myocardial perfusion scintigraphy can be automated and makes the interpretation of myocardial perfusion scintigraphy images reliable. The method of the present invention, although discussed herein in the context of myocardial imaging, may be applied to the imaging of any other organs or body parts for the purpose of diagnosis.
It is therefore a general object of the present invention to provide an improved method for myocardial imaging.
More particularly, it is an object of the present invention to provide a method for registration of myocardial images for the purpose of diagnosis.
More particularly, it is an object of the present invention to provide a method for registration of myocardial images using an elastic transformation.
An additional object of the present invention is to provide a reliable, operator independent method for the analysis and interpretation of organ images such as myocardial perfusion scintigraphies.
Another object of the present invention is to provide a method for image registration of body parts and internal organs, especially to avoid diagnosis by more invasive techniques.
Further objects and advantages of the present invention will become apparent from a consideration of the drawings and the ensuing detailed description. These various embodiments and their ramifications are addressed in greater detail in the Detailed Description.